System and method for intrusion detection using a time domain radar array

ABSTRACT

A system and method for highly selective intrusion detection using a sparse array of time modulated ultra wideband (TM-UWB) radars. Two or more TM-UWB radars are arranged in a sparse array around the perimeter of a building. Each TM-UWB radar transmits ultra wideband pulses that illuminate the building and the surrounding area. Signal return data is processed to determine, among other things, whether an alarm condition has been triggered. High resolution radar images are formed that give an accurate picture of the inside of the building and the surrounding area. This image is used to detect motion in a highly selective manner and to track moving objects within the building and the surrounding area. Motion can be distinguished based on criteria appropriate to the environment in which the intrusion detection system operates.

This application is a continuation of and claims priority under 35U.S.C. § 120 to U.S. patent application Ser. No. 09/332,502 (issued asU.S. Pat. No. 6,177,903). This application is related to U.S. patentapplication Ser. No. 9/332,503 issued as U.S. Pat. No. 6,218,979),entitled “Wide Area Time Domain Radar Array”, which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to radar motion detection, andmore particularly to using a sparse array of time modulated ultrawideband radars for highly selective intrusion detection.

2. Related Art

Today, many homes and businesses employ surveillance systems forintrusion detection. Consumers have spent billions of dollars on homesecurity systems over the last few years, and the number of homes withsecurity systems has increased by almost half. These systems varydramatically in sophistication and cost, but most include perimetersensors on outside doors and windows, motion detectors in key insideareas, a control unit to interpret and respond to signals from thesensors, and a siren or other alert mechanism. Most are connected to acentral monitoring station, which can notify the police in the eventsomething triggers one of the sensors.

Conventional intrusion detection systems, particularly those in the costrange of the average home or small business owner, suffer from very highfalse alarm rates, often 90% and above. This imposes prohibitive costson local police departments having to answer these false alarms. Manycities have responded by charging fines for answering these calls. Thisin turn provides incentive to home and business owners to deactivate thealarm system to avoid the false alarms. One study suggests that inburglarized homes with alarm systems, almost half of the alarms weren'teven activated.

Conventional intrusion detection systems suffer a high rate of falsealarms for many reasons. One reason is that these systems provideminimal selectivity. As used herein, selectivity refers to an intrusiondetection system's ability to distinguish movement on some basis, suchas where the movement is occurring, how fast an object is moving, or thepath that an object is moving along. Obviously, detection systems thatare more selective will likely suffer fewer false alarms becausethreatening movement can be more precisely defined and distinguishedfrom movement defined as benign. What is defined as threatening andbenign will vary by the particular environment in which the systemoperates. For instance, in a home environment, threatening movementcould be defined as movement around the outside perimeter of the house,while movement inside the house is defined as benign. Therefore, anintruder approaching a door or window from the outside would trigger thealarm, whereas a child opening a bedroom door would not.

A need therefore exists for a highly selective intrusion detectionsystem and method.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is directed to a system and methodfor highly selective intrusion detection using a sparse array of timemodulated ultra wideband (TM-UWB) radars. TM-UWB radars emit very shortRF pulses of low duty cycle approaching Gaussian monocycle pulses with atightly controlled pulse-to-pulse interval. Two or more of these TM-UWBradars are arranged in a sparse array (i.e., they are spaced atintervals of greater than one quarter wavelength), preferably around theperimeter of a building. Each TM-UWB radar transmits ultra widebandpulses that illuminate the building and the surrounding area. One ormore of the radars receives signal returns, and the signal return datais processed to determine, among other things, whether an alarmcondition has been triggered.

An advantage of the current invention is that ultra wideband (UWB)pulses are used. As used herein, UWB refers to very short RF pulses oflow duty cycle ideally approaching a Gaussian Monocycle. Typically thesepulses have a relative bandwidth (i.e., signal bandwidth/centerfrequency) which is greater than 25%. The ultra wideband nature of thesepulses improves both angle and range resolution, which results inimproved performance (e.g., greater selectivity, more sensitive motiondetection). The term “wavelength”, as used herein in conjunction withultra wideband systems, refers to the wavelength corresponding to thecenter frequency of the ultra wideband pulse.

Another advantage of the current invention is that high resolution radarimages are formed which give an accurate picture of the inside of thebuilding and the surrounding area. The current invention uses this imageto, among other things, detect motion in a highly selective manner andto track moving objects within the building and the surrounding area.High resolution radar images are possible because the TM-UWB radarspositioned around the perimeter of the building form a sparse arraycapable of achieving high angular resolution. Angular resolution is afunction of the width of the TM-UWB radar array, i.e., the wider thearray, the greater the angular resolution. Conventional narrowbandradars arranged in a sparse array suffer off-axis ambiguities, and aretherefore not practical. However, the UWB pulses transmitted by theTM-UWB radars are sufficiently short in duration (with very fewsidelobes) that the radars can be used in a sparse array configurationwithout off-axis ambiguities. Furthermore, range ambiguities are curedby time-encoding the sequence of transmitted TM-UWB pulses.

Another advantage of the current invention is that highly selectivemotion detection is possible. Using the high resolution radar imagesgenerated by the TM-UWB radar array, motion can be distinguished basedon criteria appropriate to the environment in which the intrusiondetection system operates. For example, home security systems accordingto the present invention can distinguish outside movement around doorsand windows from movement inside the house. Alternatively, businesssecurity systems can distinguish movement in an unsecured portion of thebuilding from movement in a secured portion. This selectivity can resultin lower false alarm rates.

Another advantage of the current invention is that high angularresolution may be achieved at a low center frequency. Because thetransmitted UWB pulses have a large relative bandwidth, and because theradar array is wide, a lower center frequency can be maintained andstill achieve a high angular resolution. Operating at a lower centerfrequency relaxes the timing requirements of the system, which makes iteasier to achieve synchronization between the radars, and results inless complex, less expensive implementations. A low center frequencyalso results in UWB pulses that are able to better penetrate lossymaterials and withstand weather effects.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements. The drawingin which an element first appears is indicated by the leftmost digit inthe corresponding reference number.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be described with reference to theaccompanying drawings, wherein:

FIG. 1 illustrates an example building environment within which thepresent invention can be used;

FIG. 2 depicts an intrusion detection system;

FIG. 3 is a flowchart that describes the operation of the intrusiondetection system;

FIG. 4 is a flowchart that describes the generation of radar images;

FIG. 5 depicts the intrusion detection system operating in a first modeincluding back scattering at each sensor and forward scattering;

FIG. 6 depicts the intrusion detection system operating in a second modeincluding back scattering at one sensor and forward scattering;

FIG. 7 depicts the intrusion detection system operating in a third modeincluding back scattering only;

FIG. 8 depicts an imaging area within an example building environment;

FIG. 9 is a flowchart that describes the generation of a radar image;

FIG. 10 depicts example reflectograms for four sensors;

FIG. 11 is a flowchart that describes processing the radar images todetermine whether an alarm condition has been triggered;

FIG. 12A depicts an example clutter map;

FIG. 12B depicts an example radar image with a moving target;

FIG. 12C depicts an example differential map, calculated as thedifference between the clutter map of FIG. 12A and the radar image ofFIG. 12B; and

FIG. 13 depicts a preferred calibration of the home intrusion system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview of the Invention

The present invention is directed to a system and method for highlyselective intrusion detection using a sparse array of TM-UWB radars.TM-UWB (or impulse) radio and radar technology was first fully describedin a series of patents, including U.S. Pat. Nos. 4,641,317 (issued Feb.3, 1987), 4,743,906 (issued May 10, 1988), U.S. Pat. No. 4,813,057(issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990)and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton.A second generation of TM-UWB patents include U.S. Pat. No. 5,677,927(issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997)and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton et al.These patent documents are incorporated herein by reference.

FIG. 1 illustrates a building environment 100 within which the presentinvention is used. The present invention includes two or more sensors102. In a preferred embodiment, four sensors 102 (102A, 102B, 102C, and102D, as shown in FIG. 1) are located around the perimeter of abuilding. Using more than four sensors 102 will further reduce the falsealarm rate. The sensors 102 communicate with each other via acommunication pathway 104. Though only a single communication pathway104 is shown, each sensor 102 can communicate with one or more of theother sensors 102.

The example building depicted in FIG. 1 includes perimeter (outside)walls 106, inside walls 112, doors 110, and windows 108. The areas inand around the building are conveniently divided into inside 114 andoutside 116. Those skilled in the art will recognize that the buildingshown in FIG. 1 is only a simple example, and that the conceptsdescribed herein apply equally well to any arbitrarily shaped building,with any configuration of doors, windows, interior walls, andfurnishings.

One of the primary objects of the present invention is to detectmovement of objects in and around a perimeter, such as outside walls ofa building. A perimeter may alternatively be defined as two boundariesto allow for noise and clutter variations. In a two boundary system, theperimeter may be defined as an inside and outside boundary separated bysome distance (e.g. 2 ft). An object on the outside would have to crossthe inside boundary to trigger an entry alarm; whereas, an object on theinside would have to cross the outside boundary to trigger an exitalarm.

The present invention will be described in an example embodiment wheremovement of object are detected in and around the building shown in FIG.1. For convenience, both an inside target 118 and an outside target 120are shown. The following discussion will refer to both collectively astargets.

FIG. 2 depicts the components of the present invention in greaterdetail, referred to collectively as an intrusion detection system 200.Each sensor 102 preferably includes a TM-UWB radar 202, and a wirelesslink 204. The sensors 102 communicate with a processor 206 that isresponsible for processing the data received by the sensors anddetermining whether an alarm condition has been met. Note that, forpurposes of clarity, only two sensors 102 (A and B) are depicted in FIG.2. As stated above, intrusion detection system 200 includes two or moresensors 102.

TM-UWB radar 202 is preferably implemented as described in U.S. Pat.Nos. 4,743,906, and 5,363,108, incorporated by reference above. However,those skilled in the art will recognize that the concepts describedherein apply equally well to other radars that transmit time modulatedUWB pulses.

TM-UWB radars 202 transmit UWB pulses and at least one receives signalreturns, depending on the particular mode of operation (describedbelow). Each TM-UWB radar 202 can utilize a single antenna element 208for both transmission and reception, separate antenna elements fortransmission and reception, or even an array of antenna elements fortransmission and reception, including phased arrays of antennas. Thoseskilled in the art will recognize that the number and type of antennaelements will vary based on the particular application and desiredtransmission characteristics.

TM-UWB radar 202 preferably operates with a center frequency between 1GHz and 3 GHz, and a pulse repetition rate of 1.25 MHZ. Other centerfrequencies are possible, though hydrometer effects introduce problemsaround 10 GHz and above. Similarly, the pulse repetition rate will varybased on the particular embodiment. Note that if the time modulation ofthe UWB pulses includes a random component, pseudo-random noise (ratherthan true noise) is used so that the noise sequence can be reproduced atthe other radars. A good discussion of time modulation usingpseudo-random noise codes for impulse systems is found in U.S. Pat. No.5,677,927 (hereafter the '927 patent), incorporated by reference above.

Sensors 102 placed along the perimeter of a building will clearly beseparated by more than a quarter wavelength at these center frequencies.The sensors therefore form a sparse array. Sparse arrays of TM-UWBradars are discussed in detail in commonly owned, U.S. patentapplication Ser. No. 09/332,503 (issued as U.S. Pat. No. 6,218,979),entitled “Wide Area Time Domain Radar Array,” which has beenincorporated by reference. Sensors 102 are preferably packaged forconvenient installation in a conventional wall electrical socket,securely fastened such that it cannot easily be removed. Those skilledin the art will recognize that three-dimensional images may be obtainedby ensuring that all the sensors 102 do not occupy the same horizontalplane, i.e., at least one sensor 102 occupies a horizontal planedifferent from the other sensors 102.

Processor 206 can be implemented using many different configurations ofcomputer hardware and software, as is well known to those skilled in theart. Each particular application will dictate the processing needs ofthe system, size requirements, memory requirements, and otherimplementational details. Processor 206 can be physically located at anyconvenient location. Processor 206 can be included in the same packagingwith a sensor 102, or close enough to a sensor such that data may betransferred between processor 206 and the nearby sensor via a cable.Alternatively, processor 206 can be physically distant from all sensor102 and communicate with one or more of them wirelessly.

Communication pathway 104 represents a wire or wireless transmissionmedium. In a preferred embodiment, sensors 102 communicate with eachother via a wireless link, wherein communication pathway 104 representselectromagnetic waves propagating through the environment.Alternatively, communication pathway 104 can be implemented as a cable(e.g., coaxial cable, optical fibre) connecting the radars.

Wireless links 204 provide for wireless communication between sensors102 via communication pathway 104. Wireless links can be implemented asany number of conventional devices known to those skilled in the art,depending upon the bandwidth required by the particular application.However, wireless link 204 is preferably implemented as a TM-UWB radio,as described in many of the above cited patents and applications. Inthis preferred embodiment, data transfers are accomplished usingsubcarrier modulation as described in the '927 patent, incorporated byreference above. Alternatively, a single TM-UWB radar can be configuredto perform the functions of wireless link 204 and TM-UWB radar 202. Inother words, a single TM-UWB radar is used at each sensor 102 totransmit UWB radar pulses and communicate wirelessly with other sensors102. Combining these functions into a single unit results in lessexpensive implementations. Further, in modes that include forwardscattering, synchronization between the radars is achieved withoutrequiring a separate synchronization signal. Note that wireless links204 are unnecessary for those embodiments employing a cable ascommunication pathway 104.

Wireless links 204 are responsible for, inter alia, transmittingscattering data received by their associated radars 202, and exchangingsynchronization information when forward scattering data is being taken.The bandwidth requirements for wireless links 204 depend upon the typesof data analysis performed by processor 206, the rate at which TM-UWBradar 202 transmits UWB pulses, and v various other factors. Wirelesslinks 204 can also be either bidirectional or simplex, depending uponthe requirements of the application. Those skilled in the art willrecognize the cost to benefit tradeoff associated with conventionalwireless implementations. Other implementations are discussed below.

Operation of the Current Invention

FIG. 3 is a flowchart that describes the operation of the currentinvention. This section provides an overview of the operation. Each stepis then described in detail in the following sections.

In step 302, intrusion detection system 200 is calibrated. Calibrationas used herein refers to, among other things, identifying the positionsof the various sensors 102 and one or more security zones. A securityzone, as described below, is an area in which certain movement couldtrigger an alarm condition. The calibration of step 302 is performedbefore intrusion detection system 200 begins monitoring buildingenvironment 100. Further details regarding calibration are providedafter detailed discussions of the next two steps.

In step 304, a radar image is generated by the operation of intrusiondetection system 200. The sensors 102 transmit UWB pulses, preferably ina omnidirectional manner, and then receive the reflected energy,referred to herein as signal returns or signal return data. Processor206 then creates a radar image based on the signal return data collectedby all sensors 102.

In step 306, processor 206 determines whether an alarm condition hasbeen met. This determination is based on the current radar image, and inmany cases, on past radar images as well. Intrusion detection system 200triggers various alarms in the event that an alarm condition is met,such as lights, sirens, and calls to emergency personnel.

The following sections described each step in detail.

Generation of Radar Images

FIG. 4 is a flowchart that describes step 304 in greater detail. In step402, flow proceeds to step 404 only for those embodiments that includeforward scattering measurements. In step 404, radars 202 aresynchronized, as described in detail below. Skilled artisans willrecognize that this synchronization allows for useful analysis of thescattering data.

In step 406, each of the radars 202 transmits UWB pulses, preferably inan omnidirectional fashion, radiating the pulsed energy in alldirections.

In step 408, signal returns are received by at least one radar 202,depending upon the mode of operation. Intrusion detection system 200preferably operates in three different modes of operation. In all threemodes, each TM-UWB radar 202 transmits UWB pulses. The different modesvary based on which radars 202 are configured to receive signal returns,and whether the radars are synchronized for forward scatteringmeasurements.

FIG. 5 depicts intrusion detection system 200 operating in a first mode.Again, for purposes of clarity, only two sensors are depicted (102A and102B) and a reflective body 502. Reflective body 502 represents anyobject, either inside 114 or outside 116, that reflects a portion of thetransmitted pulse energy. As shown, both TM-UWB radars 202 transmit UWBpulses and receive the corresponding signal returns reflecting offreflective body 502. This process is known to those skilled in the artas back scattering, or mono-static operation. The back scattering datafrom each radar 202 is passed to processor 206 (not shown in FIG. 3) foranalysis. As mentioned above, processor 206 can be located in closephysical proximity or connected wirelessly to any one or more of sensors102.

Sensors 102 also perform forward scattering (or bi-static) measurements,which refers to a TM-UWB radar 202 receiving signal returnscorresponding to UWB pulses transmitted by another sensor 102. As shownin FIG. 5, radar 202A receives signal returns corresponding to UWBpulses transmitted by radar 202B. Radar 202B passes both back andforward scattering data on to processor 206. TM-UWB radars 202 must besynchronized in order to utilize the forward scattering data. Thissynchronization is preferably implemented across communication pathway104.

Synchronizing radars 202 can be accomplished in at least two differentways. In a first embodiment, a synchronization signal is transmittedbetween radars 202 via wireless links 204. In this embodiment, wirelesslinks 204 are chosen which have high temporal resolution, on the orderof ten picoseconds. This resolution is necessary to achieve the desiredsynchronization.

In a second embodiment, each radar 202 receives UWB pulses transmittedby the radar 202B via two paths. As described above, radar 202A receivesforward scattering signal returns that reflect off reflective body 502.However, radar 202A can also receive UWB pulses that travel directlyfrom radar 202B to radar 202A. These UWB pulses can be used by radar202A for synchronization, so long as the distance between the radars isknown. Those skilled in the art will recognize that the antenna 208Bassociated with radar 202B must be chosen such that its beam patternprovides for sufficient transmission in the direction of radar 202A.

FIG. 6 depicts intrusion detection system 200 operating in a secondmode. In this mode, certain of the radars 202 are used for forwardscattering purposes only, i.e., they transmit UWB pulses which arereceived by other radars 202, but do not themselves receive any signalreturns. For example, in FIG. 6, radar 202B transmits UWB pulses thatare received by radar 202A, as indicated by the forward scatteringpropagation path. Radar 202A receives the forward scattering signalreturns corresponding to UWB pulses transmitted by radar 202B, and alsoreceives its own back scattering signal returns. If intrusion detectionsystem 200 operates only in the second mode, radar 202B can beimplemented in a more simple, inexpensive manner because it need onlytransmit, not receive.

Again, the radars must be synchronized, preferably across communicationpathway 104, in order to utilize the forward scattering data. Note thatin this mode, only the radar that receives signal returns passes data(both back and forward scattering data) to processor 206 (not shown inFIG. 6) for analysis. Furthermore, communication only needs to proceedin one direction between wireless links 204. i.e., from radar 202A toradar 202B. Therefore, for embodiments only operating in the secondmode, wireless link 204B can be implemented as a receiver only.

FIG. 7 depicts intrusion detection system 200 operating in the thirdmode. In this mode, all of the radars 202 collect back scattering dataonly. As shown in FIG. 7, each radar 202 transmits UWB pulses andreceives the corresponding signal returns. The back scattering datacollected by each radar 202 is passed on to processor 206 (not shown inFIG. 7) for analysis. Note that in this mode, there is no requirementthat the radars 202 be synchronized because forward scattering data isnot being collected.

Returning to the flowchart of FIG. 4, in step 410, processor 206generates a radar image based on the signal return data collected bysensors 102. FIG. 8 depicts building environment 100 for purposes ofillustrating the analysis of back scattering data (and forwardscattering, where available) to generate an image of inside target 118.FIG. 8 also depicts an imaging area 802 that defines an example area tobe imaged. Imaging area 802 could, for example, represent a portion ofthe building inside 114, the entire inside 114, or the inside 114 andoutside 116. The needs of each particular intrusion system willdetermine which areas require surveillance, i.e., radar imaging.

A grid 804 crisscrosses imaging area 802, defining one or more voxels806 (a voxel is a minimum resolution portion of a three dimensionalspace, comparable to a pixel in two dimensional space). As describedbelow, processor 206 calculates a value for each voxel 806 indicative ofthe reflected energy measured in the portion of imaging area 802 definedby that voxel. The resulting grid 804 of voxels 806 forms a radar imageof imaging area 802. Grid 804 is maintained in processor 206, and canvary in spacing to define voxels 806 having different resolution (grid804 need not be orthogonal). Decreasing the grid spacing increases theresolution of the generated image. As shown in FIG. 8, inside target 118occupies a single voxel 806A. Though this simplifies the discussion,skilled artisans will recognize that in practice a higher resolutionwill often be desired.

FIG. 9 is a flowchart that depicts step 410 in greater detail accordingto a preferred time domain interferometry technique for calculating avalue for each voxel 806 in imaging area 802. In step 902, areflectogram is generated for each radar 202 in intrusion detectionsystem 100. FIG. 10 depicts four example reflectograms, 1002, 1004,1006, and 1008, corresponding to sensors 102A, 102B, 102C, and 102D,respectively. Skilled artisans will recognize that a reflectogramdescribes reflected energy as a function of range (i.e., distance fromthe transmitting antenna). For example, reflectogram 1002 describes thereflected energy measured at sensor 102A, whereas reflectogram 1004describes the reflected energy measured at sensor 102B. The x-axisrepresents range, while the y-axis represents reflected energy measuredas voltage.

In a preferred embodiment, each radar 202 generates a reflectogram bysweeping through the ranges of interest, measuring reflected energy atdiscrete ranges. At each discrete range, radar 202 transmits ultrawideband pulses 808 and then looks for reflected energy after a timedelay corresponding to the return time-of-flight. Further detailsregarding the operation of radar 202 are provided in U.S. Pat. Nos.4,743,906, and 5,363,108, incorporated by reference above. Radar 202receives and, where multiple pulses are transmitted for each discreterange step, accumulates reflected energy.

Those skilled in the art will recognize that more reflected energy willbe measured per transmitted pulse for nearby targets, as compared tothose targets positioned farther away. Compensating for this effectallows for more efficient use of the radar's dynamic range. In apreferred embodiment, radar 202 transmits and receives an increasingnumber of pulses per discrete range step as the range is increased. Thereflected energy measured at longer ranges is therefore increased byreceiving and integrating a greater number of pulses. The ranges ofinterest are preferably divided into multiple “range windows,” where thesame number of pulses is transmitted for each discrete range within agiven window. Skilled artisans will recognize that this is only oneexample of how this compensation might be implemented.

Alternatively, the power of transmitted pulses can be varied accordingto range. In this embodiment, radar 202 increases the power oftransmitted pulses as the range gets longer. This alternativecompensation has a similar effect to varying the number of transmittedpulses, but will likely require more costly modifications to the basicradar 202 to implement. This, and other related concepts are describedin commonly owned, co-pending U.S. patent application Ser. No.09/332,501, filed Jun. 14, 1999, entitled “System and Method for ImpulseRadio Power Control,” which is incorporated herein by reference.

Returning again to FIG. 9, in step 904 an image is formed by selectivelycombining data from the reflectograms generated in step 902. An imagevalue is calculated for each voxel 806, where the image value isindicative of the total amount of reflected energy measured over thatportion of imaging area 802. Processor 206 preferably calculates animage value for each voxel 806 by summing voltage values from thereflectogram associated with each sensor 102, where the voltage valuescorrespond to the return time-of-flight from the radar to the voxelbeing calculated. For example, referring to FIGS. 8 and 10, the imagevalue for voxel 806A is the sum of a voltage value from reflectograms1002,1004, 1006, and 1008 corresponding to the return time-of-flight. Asshown in reflectogram 1002, the voltage value at time t1 corresponds tothe return time-of-flight from sensor 102A to voxel 806A, as shown inFIG. 8. Similarly, times t2, t3, and t4 correspond to the returntime-of-flight from sensors 102B, 102C, and 102D to voxel 806A, as shownin reflectograms 1004, 1006, and 1008. The sum of these four valuesforms the image value for voxel 806A.

In this manner the image value for each voxel 806 in image area 802 iscalculated as the sum of a voltage from each reflectogram correspondingto the return time-of-flight.

Intrusion Detection

Returning to FIG. 3, in step 306, processor 206 determines whether analarm condition has been triggered indicating an intrusion. What isdefined as an alarm condition depends upon the particular environment inwhich intrusion detection system is used. For example, in a homesecurity environment, an alarm condition is triggered when a movingobject approaches and penetrates a perimeter around the outside of thehouse or some other predetermined exterior boundary. Alternatively, in abuilding security environment, movement in a restricted area within thebuilding triggers an alarm condition. Those skilled in the art willrecognize that alarm conditions will vary, depending upon the exactenvironment in which intrusion detection system 200 is installed and thetypes of intrusion that are to be detected.

In a preferred embodiment, processor 206 uses the radar images generatedin step 304 to detect motion and to track moving objects. In manyinstances, processor 206 need only detect movement in a given area. Inthe aforementioned building security environment, movement detected in arestricted area triggers an alarm condition. Other alarm conditionsrequire additional processing to distinguish between different types ofmovement. For instance, movement in the vicinity of a window shouldtrigger an alarm condition if the object approached the window fromoutside 116, but not if the object approached from inside 114. Processor206 can distinguish between these two types of movement by trackingmoving objects over time.

FIG. 11 is a flowchart that depicts step 306 in detail according to apreferred embodiment. In step 1102, processor 206 updates a clutter map.The clutter map represents stationary and other “don't care” objectswithin imaging area 802. For instance, a clutter map might includestationary objects such as furniture and walls within a building. Theclutter map can also include moving objects that should not trigger analarm condition, such as ceiling fans.

Those skilled in the art will recognize that the clutter map can bedetermined in different ways. In one embodiment, the first radar imagegenerated by intrusion detection system 200 is defined as the cluttermap. This approach is easy to implement, but is not very robust. Forinstance, if a piece of furniture within imaging area 802 is moved afterthe clutter map is generated, it will thereafter appear as a movingobject because it was not part of the clutter map. In this embodiment,processor 206 sets the clutter map equal to the first radar imagegenerated in step 304, and does not change the clutter map based onsubsequent radar images.

In a preferred embodiment, however, the clutter map is updated based onsubsequent radar images by low-pass filtering the current radar image ona voxel by voxel basis, and adding the filtered image to the storedclutter map. In this way, the clutter map is slowly updated over time sothat stationary objects not present initially will be incorporated intothe clutter map. For example, if sensors 102 transmit UWB pulses with acenter frequency of 2 GHz, and if the 3 dB knee of the lowpass filter is0.1 Hz, then anything moving at a rate faster than ¾ inches in 10seconds will not be passed through the lowpass filter to the cluttermap.

In step 1104, processor 206 subtracts the updated clutter map from thecurrent radar image. The resulting image represents objects withinimaging area 802 that were not present in past radar images. FIG. 12Adepicts an example clutter map 1200 of building environment 100,including stationary objects such as doors 110, windows 108, interiorwalls 112 and exterior walls 106 (assume that everything shown in FIG.12A is within imaging area 802). FIG. 12B depicts a radar image 1202generated subsequent to clutter map 1200. As shown, inside target 118has entered the building. FIG. 12C depicts a differential map 1204calculated in step 1104 by subtracting clutter map 1200 from radar image1202. Differential map 1204 therefore represents objects that have movedwithin imaging area 802. The appearance of inside target 118 willtrigger an alarm condition for those intrusion detection systems thatare configured to detect movement in that particular area.

In step 1106, a track file is updated based on differential map 1104calculated in step 1104. The track file contains information on movingobjects being tracked within imaging area 802. For example, a track fileis a collection of historical information on identified objects to allowdetermination of object motion parameters, such as position, speed,velocity, and direction. In a preferred embodiment, objects that appearin differential map 1204 are compared against those objects currentlybeing tracked in the track file. Each object in differential map 1204 iseither associated with and used to update an existing object in thetrack file, or is added to the track file as a new object to track.

One method of generating a track file is to map an area usingreflectogram data from several sensors, and then later, map the areaagain and subtract the first map data to derive a map of changesrelating to motion in the area. The largest peaks are then identified asobjects to be tracked and all energy within a radius (e.g., 1 foot) ofeach peak is considered part of the object. The object centroid is thenfound by determining the centroid of all of the “change” signal withinthe radius. This set of centroids is then compared with previouscentroids from the track file. The nearest previous object would beconsidered the same object of the purposes if determining object motion,velocity, direction. These parameters may be determined from the historyof the object centroid locations.

A track file may alternatively be maintained by determining an areawithin some range (e.g., 1 foot) of a previous centroid location for anobject, and then computing a new centroid based on this area to beassociated with the object. In this way, an object may be incrementallytracked across a room and objects may be determined as entering orexiting a door or widow.

Map threshold levels may be used to limit the number of objects to areasonable level. Objects may disappear, or be dropped from the trackfile, if the total energy drops below a disappearance threshold for aperiod of time. Likewise objects may be generated based on a single peakthreshold crossing, but may not achieve full “object” status until itmaintains threshold for a period of time.

Tracking the movement of objects within imaging area 802 allows for moresophisticated alarm conditions to be defined. For instance, in the homesecurity environment described above, an alarm condition might betriggered where outside target 120 approaches window 108, whereas insidetarget 118 approaching window 108 does not. Those skilled in the artwill recognize the many ways that tracking could be used to definerobust alarm conditions in a variety of environments.

Calibration

Returning to FIG. 3, intrusion detection system 200 is calibrated instep 302 prior to generating a radar image in step 304 and detectingintrusion in step 306. The processing described above with respect tosteps 304 and 306 depends, in part, on having accurate knowledge ofwhere the sensors are located with respect to one another. Calibratingintrusion detection system 200 refers to determining these relativepositions.

FIG. 13 depicts a first alternative calibration system for intrusiondetection system 200. A portable transmitter 1302 is moved along acalibration path 1304 around the area to be protected. For the exampleshown in FIG. 13, calibration path 1304 follows the outside walls 106 ofthe building. Those skilled in the art will recognize that calibrationpath 1304 will vary for different environments and alarm conditions.Portable transmitter 1302 transmits UWB pulses, such as a TM-UWB radar202.

All of the sensors 102 lock their receivers to transmitter 1302 andtrack its movement around calibration path 1304. As the sensors 102track transmitter 1302, datum marks are made periodically. This ispreferably accomplished by the operator pressing a button that modulatestransmitter 1302, sending a bit stream to each sensor 102 identifyingthe index number of the data point being sent. Alternatively, areal-time clock can be used to continually mark the data received by thesensors 102. In either case, after completion each sensor 102 sends thecalibration data to processor 206 to determine the position of thesensors 102 in relation to each other and calibration path 1304.

In a second alternative embodiment, in step 302, the calibration isperformed manually, by locating each sensor on a map, blueprint, survey,or by direct measurement. The calibration data is entered into processor206 by conventional means familiar to those skilled in the art.

In a third alternative embodiment, in step 302, each sensor 102 locks onto UWB pulses transmitted by another sensor 102, one after another,until a range is determined between each pair of sensors 102. Thesensors can be adapted to perform range finding as described in commonlyowned, co-pending U.S. patent application Ser. No. 09/045,929, attorneydocket no. 1659.0470000, filed Mar. 23, 1998, entitled “System andMethod For Position Determination By Impulse Radio,” which isincorporated herein by reference. Another alternative embodiment foradapting the sensors to perform range finding is described in commonlyowned, co-pending U.S. patent application Ser. No. 09/083,993, attorneydocket no. 1659.0660000, filed May 26, 1998, entitled “System and MethodFor Distance Measurement by Inphase and Quadrature Signals In A RadioSystem,” which is also incorporated herein by reference. Each sensor 102sends the calibration data to processor 206 to determine the position ofthe sensors 102 in relation to each other.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method for detecting motion of an object usingat least first and second ultra wide band (UWB) radars that areseparated from one another, comprising the steps of: (a) receiving firsttime modulated UWB (TM-UWB) pulses at the first and second UWB radars,the first TM-UWB pulses generated by an UWB transmitter moving along acalibration path, the calibration path defining an area to be monitored;(b) determining positions of the first and second UWB radars in relationto each other and the calibration path based on the first TM-UWB pulsesreceived at the first and second UWB radars; (c) transmitting secondTM-UWB pulses from the first UWB radar; (d) transmitting third TM-UWBpulses from the second UWB radar; (e) receiving signal returns at atleast one of the first and second TM-UWB radars; and (f) detectingmotion within the area to be monitored based on the received signalreturns.
 2. The method of claim 1, wherein said step (b) comprises:(b.1) generating calibration data based on the first TM-UWB pulses- and(b.2) determining the positions of the first and second UWB radars inrelation to each other and the calibration path on the calibration data.3. The method of claim 1, further comprising g a step of generating animage based on the signal returns.
 4. The method of claim 1, whereinsaid step (f) comprises the steps of: (f.1) generating reflectogram databased on the signal returns; (f.2) generating an image based on thereflectogram data; and (f.3) detecting motion based on the image.
 5. Themethod of claim 1, wherein said step (f) comprises: (f.1) generating animage based on the signal returns; (f.2) subtracting the image from aclutter map to thereby create a differential map; and (f.3) detectingmotion based on the differential map.
 6. The method of claim 5, furthercomprising a step of triggering an alarm when motion is detected, andwherein the clutter map represents objects that should not cause thetriggering of the alarm.
 7. The method of claim 1, wherein said step (a)further comprises moving the UWB transmitter along the calibration path.8. The method of claim 1, wherein said step (e) comprises receivingsignal returns at both the first and second UWB radars.
 9. A system fordetecting motion of an object, comprising: a first ultra wide band (UWB)radar adapted to receive first time modulated UWB (TM-UWB) pulses froman UWB transmitter moving along a calibration path defining a region tobe monitored, said first UWB radar also adapted to transmit secondTM-UWB pulses; a second UWB radar separated from said first UWB radar,said second UWB radar adapted to receive said first TM-UWB pulses fromthe UWB transmitter moving along the calibration path, said first UWBradar also adapted to transmit third TM-UWB pulses; and a processor incommunications with said first and second UWB radars; wherein said firstand second UWB radars are also adapted to determine calibration datafrom the first TM-UWB pulses, to receive signal returns, and to forwardthe calibration data and signal return data to said processor, whereinsaid processor is adapted to determine positions of said first andsecond UWB radars in relation to each other and the calibration pathbased on the calibration data, and to detect motion within the region tobe monitored based on the signal return data.
 10. The system of claim 9,wherein the region includes a boundary having a first side and a secondside, and wherein said processor triggers an alarm when it detectsmotion toward the first side of the boundary.
 11. The system of claim10, wherein the boundary is defined by a location of a window.
 12. Thesystem of claim 10, wherein the region includes a predeterminedboundary, and wherein said processor triggers an alarm when saidprocessor detects motion toward, and penetration of, the boundary by anobject.
 13. The system of claim 10, wherein the boundary includes afirst side and a second side, and wherein said processor only triggersthe alarm when said processor detects motion toward, and penetration of,the first side of the boundary.
 14. The system of claim 13, wherein theboundary is defined by a location of a window.
 15. The system of claim14, wherein the first side of the boundary is outside a building, andwherein the second side of the boundary is inside the building.
 16. Thesystem of claim 9, wherein the processor generates an image based on thesignal returns and detects motion based on the image.
 17. The system ofclaim 9, wherein said processor generates one or more reflectogramsbased on the signal returns and generates the image based on the one ormore reflectograms.
 18. The system of claim 9, wherein said processorgenerates an image based on the returns signals, subtracts the imagefrom a clutter map to thereby create a differential map, and detectsmotion based on the differential map.
 19. The system of claim 18,wherein said processor triggers an alarm when it detects motion, andwherein the clutter map represents objects that should not cause theprocessor to trigger the alarm.